This invention relates to improved methods for making composite heat sources. The heat sources made by the methods of this invention are particularly suitable for use in a smoking article such as that described in Commonly assigned U.S. Pat. No. 4,991,606. The composite heat sources have low ignition and high combustion temperatures that generate sufficient heat to release a flavored aerosol from a flavor bed for inhalation by the smoker. Upon combustion, the composite heat sources produce virtually no carbon monoxide.
According to the methods of this invention, a metal oxide, a fully reduced metal or combination of these is mixed with a carbon source. Upon heating, the mixture is, in part, converted to metal species. As used herein, metal species is meant to include metal carbides, metal oxides and/or the fully or partially reduced metal which arises during preparation of the composite heat source. In a preferred embodiment, the metal oxide/carbon source mixture is pre-formed into a desired shape and converted to metal species in situ, without substantially altering the shape of the mixture.
There have been previous attempts to provide a heat source for a smoking article. While providing a heat source, these attempts have not produced a heat source having all of the advantages of the present invention.
For example, Siegel U.S. Pat. No. 2,907,686 discloses a charcoal rod coated with a concentrated sugar solution which forms an impervious layer during burning. It was thought that this layer would contain gases formed during smoking and concentrate the heat thus formed.
Ellis et al. U.S. Pat. No. 3,258,015 and Ellis et al. U.S. Pat. No. 3,356,094 disclose a smoking device comprising a nicotine source and a tobacco heat source.
Boyd et al. U.S. Pat. No. 3,943,941 discloses a tobacco substitute which consists of fuel and at least one volatile substance impregnating the fuel. The fuel consists essentially of combustible, flexible and self-coherent fibers made of a carbonaceous materials containing at least 80% carbon by weight. The carbon is the product of the controlled pyrolysis of a cellulose-based fiber containing only carbon, hydrogen and oxygen.
Bolt et al. U.S. Pat. No. 4,340,072 discloses an annular fuel rod extruded or molded from tobacco, a tobacco substitute, a mixture of tobacco substitute and carbon, other combustible materials such as wood pulp, straw and heat-treated cellulose or a sodium carboxymethylcellulose (SCMC) and carbon mixture.
Shelar et al. U.S. Pat. No. 4,708,151 discloses a pipe with replaceable cartridge having a carbonaceous fuel source. The fuel source comprises at least 60-70% carbon, and most preferably 80% or more carbon, and is made by pyrolysis or carbonization of cellulosic materials such as wood, cotton, rayon, tobacco, coconut, paper and the like.
Banerjee et al. U.S. Pat. No. 4,714,082 discloses a combustible fuel element having a density greater than 0.5 g/cc. The fuel element consists of comminuted or reconstituted tobacco and/or a tobacco substitute, and preferably contains 20-40% by weight of carbon.
Published European patent application 0 117 355 by Hearn et al. discloses a carbon heat source formed from pyrolized tobacco or other carbonaceous material such as peanut shells, coffee bean shells, paper, cardboard, bamboo, or oak leaves.
Published European patent application 0 236 992 by Farrier et al. discloses a carbon fuel element and process for producing the carbon fuel element. The carbon fuel element contains carbon powder, a binder and other additional ingredients, and consists of between 60% and 70% by weight of carbon.
Published European patent application 0 245 732 by White et al. discloses a dual burn rate carbonaceous fuel element which utilizes a fast burning segment and a slow burning segment containing carbon materials of varying density.
These heat sources are deficient because they provide unsatisfactory heat transfer to the flavor bed, resulting in an unsatisfactory smoking article, i.e., one which fails to simulate the flavor, feel and number of puffs of a conventional cigarette. Commonly assigned U.S. Pat. No. 5,076,296, solved this problem by providing a carbonaceous heat source formed from charcoal that maximizes heat transfer to the flavor bed, releasing a flavored aerosol from the flavor bed for inhalation by the smoker, while minimizing the amount of carbon monoxide produced.
However, all conventional carbonaceous heat sources liberate some amount of carbon monoxide gas upon ignition. Moreover, the carbon contained in these heat sources has a relatively high ignition temperature, making ignition of conventional carbonaceous heat sources difficult under normal lighting conditions for a conventional cigarette.
Attempts have been made to produce non-combustible heat sources for smoking articles in which heat is generated electrically, e.g., Burruss, Jr., U.S. Pat. No. 4,303,083, Burress U.S. Pat. No. 4,141,369, Gilbert U.S. Pat. No. 3,200,819, McCormick U.S. Pat. No. 2,104,266 and Wyss et al. U.S. Pat. No. 1,771,366. These devices are impractical and none has met with any commercial success.
Attempts have been made to produce a combustible, non-carbonaceous heat source. Commonly assigned U.S. Pat. No. 5,040,522, relates to such a heat source. Although combustion of the non-carbonaceous heat source yields up to tenfold less carbon monoxide than combustion of conventional carbonaceous heat sources, some carbon monoxide is still produced. Moreover, the method of producing the heat source disclosed in that application requires separate steps to produce the metal carbide and to form it into suitable shape for use as a heat source. Co-pending U.S. patent application Ser. No. 07/443,636, filed on Nov. 29, 1989, and commonly assigned herewith, relates to a metal nitride heat source that also produces substantially no carbon monoxide or nitrogen oxides upon combustion. Co-pending U.S. patent application Ser. No. 07/556,732, filed on Jul. 20, 1990 and commonly assigned herewith, is directed towards a heat source comprising carbon and metal carbide that also produces substantially no carbon monoxide upon combustion. Attempts have been made to produce pyrophoric materials comprising metal aluminides for use as a decoy for heat-seeking missiles, e.g., Baldi, U.S. Pat. No. 4,799,979. These devices, however, combust too rapidly and produce too intense a heat to be used as a heat source in a smoking article.
There have been previous attempts to prepare iron carbide. Grey et al. U.S. Pat. No. 3,885,023 and Okamura et al. published European patent application 0 180 162 disclose the preparation of iron carbide particles by reducing iron oxide in a carbon monoxide atmosphere. Stelling et al. U.S. Pat. No. 2,780,537, Okamura U.S. Pat. No. 4,842,759 and Shibuya et al. published European patent application 0 123 318 disclose the preparation of iron carbide particles by reducing iron oxide in a carbon monoxide/reducing gas mixture. Rogers U.S. Pat. No. 3,572,993 discloses the preparation of ultrafine iron carbide particles by reducing iron carbonyl in a carbon monoxide/hydrogen atmosphere.
Additionally, metal carbides may be prepared by reduction of the metal oxide with elemental carbon; carbidization of the metal or metal oxide with a gaseous species such as methane, ethane, ethylene or propane; and direct reaction of the fully reduced metal with elemental carbon. (Darken, L. S. and Gurry, R. W., Physical Chemistry Of Metals, McGraw Hill, New York (1953); Storms, K., The Refractory Carbides, Academic Press, New York (1967)).
Most known methods of preparing iron carbide generally require an atmosphere that reduces and carbidizes the precursor to the metal carbide. These gases are highly explosive and/or toxic and safety precautions must be taken when using them. Other shortcomings of known methods are their high capital and production costs. The gaseous reagents employed require expensive manifolds for the control of reaction conditions and the disposal or recovery of reagents. Moreover, when these methods are used, control of end-product composition is difficult. The use of a reducing/carbidizing atmosphere in these methods may result in polymorphous metal carbide containing carbon deposits, which may upon combustion, incompletely oxidize resulting in the generation of carbon monoxide, albeit at lower levels than in carbonaceous heat sources.
Finally, the metal carbide produced by these prior methods is in particulate form and must be formed into a shape suitable for use as a heat source. Metal carbides are by nature brittle, intractable materials, which, once formed, are difficult and expensive to form into a desired shape.
It would be desirable to provide a method for producing a composite heat source that does not require the use of dangerous gaseous reagents.
It would further be desirable to provide a method of producing a composite heat source at low capital and production costs.
It would be desirable to provide a method for producing a composite heat source which allows for control of end-product composition.
It would also be desirable to provide a composite heat source which is stable at ambient temperatures and humidity.
It would further be desirable to provide a method of producing a composite heat source in which the starting materials are pre-formed into a desired shape and converted in situ to a heat source containing carbon and metal species.
It would be desirable to provide a composite heat source that liberates virtually no carbon monoxide upon combustion even though the heat source contains a significant amount of carbon.
It would also be desirable to provide a composite heat source that has a low ignition temperature to allow for easy lighting under conditions typical for a conventional cigarette, while at the same time having a combustion temperature high enough to provide sufficient heat to release flavors from a flavor bed.
It would further be desirable to provide a composite heat source that does not self-extinguish prematurely.